As MxxCon pointed out this is part of the integrity checking performed by FlashFXP to insure the best file transfer rule is used.
Customers who do not require this level of integrity checking can turn off this feature, however please be aware that if the cache is stale (i.e. other users on the server modifying files since the folder was cached) that FlashFXP cannot guaranty the best file transfer rule will be applied.
This feature can be turned off globally via Options > File Transfer Rules
On the File Transfer Rules dialog uncheck Request file size/date prior to transfer and click OK,
This setting can also be changed per-site via the Site Manager
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