Without your configuration or additional details, cannot kay exactly why you experience slow performance. At minimum provide below output to see
AAP1#sho dot11 associations <client_mac_addr>
I would look for the following to begin with.
1. Configured channel width on 5GHz radio
2. Make sure AES encryption enable (not TKIP)
3. WMM is enabled
4. No other APs nearby operate on same channel
5. Client capability (spatial stream, 11n/11ac)
There are many posts on the cisco community talk about this issue, pls go through some of them listed below.
https://community.cisco.com/t5/wireless/autonomous-3500i-slow-speeds/td-p/4073821
https://community.cisco.com/t5/wireless/aironet-1602i-standalone-slow-transfer-speed/td-p/2491665
https://community.cisco.com/t5/wireless/slow-speed-between-two-3602-access-points-configured-as/m-p/3709295
HTH
Rasika
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