wifi speed loss of 50%-70% only on this laptop

wifi speed loss of 50%-70% only on Toshiba C55D-A5120

well i am having a issue, ive tryed everything know to man to fix the issue. i pay for 60/4 internet...until here recently im only getting 4mbs-30mbs download speed, and my ping is +- 30 over what it usually is, i can be a couple inches away from the router and only have 51% signal (-54 dBm). ive reset my os 3+ times and reconfigured router numerous times. ive set to all channels, no help all the same. all my other devices get FULL speed. i can bring my laptop to my parents charter, and it will still not get FULL speeds. i had amd quick stream, i seen were it was alot of peoples issues but not mine.... im starting to believe its the adapter, i bought one for $5 not even a few hours ago off ebay, hoping its the culprit. no firewall is on, all drivers have been updated, reinstalled. and updated with no luck (( if anyone has any info on what i may be doing wrong would be very greatfull!!