Copy lag and slippage: why copying a profitable trader can still lose

The gap between a trader’s fill and yours is where most copy-trading results are decided. What creates it, how to measure it, and which traders survive it.

The most common way to lose money copying a winner is not picking the wrong winner. It is arriving late. Between the moment a trader fills and the moment your order reaches the book, the price has already responded — and on prediction markets, where a share can only end at $0 or $1, that gap is not noise. This page explains where the delay comes from, why it is not the same as ordinary slippage, and how to tell whether a trader’s edge survives it.

Two different costs, often confused

Slippage is the difference between the price you expected and the price you got — it exists for every trader, copying or not, and it grows with order size and thin books.

Copy lag is different: it is the price movement caused by the very trade you are following, plus everyone else reacting to it, before your order exists. Slippage is friction. Copy lag is being second.

You can reduce slippage by trading smaller or setting a tighter tolerance. You cannot reduce copy lag at all — you can only choose traders and markets where it costs less.

Where the delay comes from

Each step is small. Together they are enough that on a market moving in seconds, the price you copy at may be several cents away from the price the trader got — on an instrument that pays out one dollar.

• Settlement: the trade must be confirmed on-chain before it is public.
• Feed: the public activity API polls, so there is a gap between confirmation and visibility.
• Decision: the copier checks limits, risk gates and available size.
• Execution: your order has to reach the book and find a counterparty.

Why a few cents decides everything

On a stock, buying 1% worse than someone else costs you 1% of the move. On a prediction market, the payoff is fixed at $1. Buying at $0.66 instead of $0.62 does not shave your profit — it removes about 12% of the upside you were buying, because the distance to $1 shrank from 38 cents to 34.

That is why a trader can be genuinely, repeatedly right and still be uncopyable: their edge is smaller than the price move their own trades cause.

How to measure whether the edge survives

The honest measurement compares two numbers on the same trades: what the trader earned, and what a copier filling after them would have earned. If the second is close to the first, the edge survives the delay. If it collapses, the trader is a good trader and a bad source of copies.

This has to be measured forward, on live prices, not replayed on history. A historical replay silently assumes you could have filled at the old price — which is exactly the assumption in question.

Which traders survive the lag

• Slow markets: questions resolving in weeks, where a few minutes of delay changes little.
• Positions built gradually rather than in one decisive strike.
• Liquid markets, where an order does not walk the book on its own.
• Edges based on judgement about the outcome rather than on speed of reaction to news.

What to do with the number

Once you can see copy-adjusted results, most leaderboards become readable in a different way. The largest absolute earners are often the least copyable, because size and speed are exactly what creates the gap. The traders worth following tend to look unspectacular: moderate returns, shallow drawdowns, slow markets, and results that barely change when you add the delay.

Common questions

What is a normal copy lag on Polymarket?

It varies by market rather than being a single figure. On weeks-long questions the delay is practically irrelevant; on markets resolving within minutes it can be the difference between profit and loss. The useful number is not the delay in seconds but the difference in outcome it produces.

Can I avoid copy lag with a faster bot?

Only partly. Some of the delay is on-chain settlement and public feed latency, which no client can skip — the trade is invisible until it is public. Speed helps at the margins; choosing copyable traders helps far more.

Is a wider slippage tolerance a good idea when copying?

It raises the share of trades that go through and lowers the quality of the ones that do. A tight tolerance skips more but only fills near the intended price. Neither is right in general — it depends on whether the trader’s edge is large enough to survive a worse fill.

Does copy lag apply to exits as well as entries?

Yes, and it is often worse. Exits cluster: when a well-followed trader closes, everyone copying them tries to close at once, into a book that has just seen the original sale.

Based on how PolyCopy evaluates traders: every trader is paper-copied in real time against live prices, and the headline metric is the return a copier would have achieved after the delay, not the trader’s own PnL.

Originally published at poly-copy.net.