Answer
IFS Applications enforces a maximum possible Output Channel row count of 100,000 on Apps10 (with a lower default limit of 10,000, adjustable in Tools > Options), because Output Channel is fundamentally designed for a limited row count, and performance degrades sharply well before that ceiling on large datasets. Separately, an overview screen's grid only fetches a small initial batch of rows, roughly 103, at a time as you scroll — if you output 'all rows' from a screen whose full result set hasn't already been fetched into the client, the output process itself has to perform that same slow incremental fetching in the background, which explains the initial slow run. Pressing Ctrl+End on the overview screen first, to force-fetch the entire result set into the client before running the output, commonly explains the dramatic speed difference on an immediate rerun, since the data is now already fully loaded client-side rather than needing to be fetched again in small batches. For genuinely large row counts, above roughly 100,000, don't use Output Channel at all — use Quick Report's Export to Excel instead, which handles large datasets considerably better.