Thanks for the feedback

The channel change is meant to avoid interferences from other wireless networks and/or other equipement (cordless phone etc) i believe.

The WEP change is because the encryption is apparently slowing down the whole process. With basically the data transiting at the same speed, but a bottleneck in decryption or something like that. I'll try with zero security on for a little while, to see if it's a big impact.

Drilling unfortunately won't help, i would need to dig a tunnel instead (crossing 2 rooms and a corridor)
Actually, I might have a look at what's under the house and drill vertically instead of horizontally.