When there are many files to be transferred, and small ones at that, this has been reported
Consider PC to mobile transfer app, which uses FTP (best suited for for your purpose as against MTP ) to transfer on WiFi or Mobile data. There are other apps too, but this worked well for me in the past:
Share Via http is anotheran open sourceapp app worth considering. It's handy to share with other mobile devices as well
Alternatively, you can consider other File SyncFile Sync apps from the curated list maintained by Izzy