slow Wi-Fi speed after updating to High Sierra

[idboy]

Hi
iam just new mac user and rencently update my MBP 2017 to high sierra but after update my Wi-fi drop constantly and when i try to connect again my wifi take longer time to connect and when wifi is already connect my speed become much slower....i already try fix method from OSX Daily but it seem's not working
please help me !!

sorry for bad Engglish
iam from Bali Indonesia
this is sample before and after update

 

[h9826790]

Are you sure you were testing the same Wifi network? (e.g. not 2.4GHz vs 5GHz)

 

[idboy]

Are you sure you were testing the same Wifi network? (e.g. not 2.4GHz vs 5GHz)

Thanks for your feedback
I not sure but I currently using an public wifi i don't know the faster one is 5g channel or 2g channel but the point is what should I do to restore my speed again? Should I try fresh install again? I assume that problem because software fault I already try with my old windows laptop too.. and work fine on the same networks
Unlike Windows Mac can't switch preferring wifi band channels like Windows via device manager