You need to identify what the difference in speed is between a wireless and wired connection inside your own property, ideally to the same device (not at the same time). If there's no difference then the throttling is down to contention in your area or other issues with your ISP.
If there is a difference (and it's almost certainly the wired connection that will be faster), then your home wifi setup is falling back to a lower speed connection between the WAP (wireless access point) and the PC - various reasons, usually down to signal strength / interference from nearby WAPs.
If you don't need to support older wifi - b, g for example - then you should switch off support in your WAP for the unneeded bands, as this certainly used to be a cause of reduced throughput.
If you rarely switch off your router, then you would expect the DHCP lease to remain fairy static, irrespective of the lease period. That's how DHCP works - the client attempts to renew the lease before the expiration time (usually at 50% of the lease period). Since the address is palpably still in use (and therefore not in use by anything else) the client should not have a problem renewing it and there would be no need for it to change, in normal circumstances.
SD