It will be nearly indistinguishable from running the SSD on a 6 Gbps SATA 3 port.
Sequential speeds, time to read 1 GB of data
HDD = about 125 MB/s = 8 sec
SATA 2 SSD = 250 MB/s = 4 sec (4 sec faster)
SATA 3 SSD = 500 MB/s = 2 sec (6 sec faster)
So the SSD in SATA 2 gives you 67% the wait time reduction of switching from a HDD to SATA 3 SSD.
4k speeds, time to read 100 MB of data
HDD = about 1 MB/s = 100 sec
SATA 2 SSD = 40 MB/s = 2.5 sec (97.5 sec faster)
SATA 3 SSD = 40 MB/s = 2.5 sec (97.5 sec faster)
So the SSD in SATA 2 gives you the same wait time reduction of a SATA 3 SSD
Mixed 1 GB sequential and 100 MB 4k data
HDD = 108 sec
SATA 2 SSD = 6.5 sec (101.5 sec faster)
SATA 3 SSD = 4.5 sec (103.5 sec faster)
So the SSD in SATA 2 mode will give you 98% the wait time reduction of being plugged into a SATA 3 port.
Basically the only time most people would notice the speed difference is when copying large media files from one folder to another on the SSD. In other regular tasks (where the delays due to small file reads dominate), SATA 2 is nearly indistinguishable from SATA 3.
https://www.tomshardware.com/reviews/sata-6gbps-performance-sata-3gbps,3110-7.html
The only gotcha with SSDs on SATA 2 is that some SATA 2 implementations didn't enable NCQ by default, which allows multiple small file read/writes to be queued and processed together. This can improve 4k read speeds from about 35 MB/s and 4k write speeds from 70 MB/s to around 300 MB/s. If your SATA 2 ports have and are being used in AHCI mode, then NCQ is enabled (a lot of them defaulted to being used in IDE mode, I guess for compatibility with older OSes).