The router is 10 years old already, but if it used to work fast before tt might be that the chip is overheating which results to throttling.
My suggestion is to upgrade to at least an 802.11n router, preferably with gigabit ethernet. Any decent 802.11n router would give far better speed with your home network (not internet speeds). A 802.11n router with Gigabit Ethernet switch can also provide that 100 MB/s speed that you get with Ethernet crossover cable. 802.11ac will only help with WiFi speeds, but if your devices aren't 802.11ac, it would be a waste of money to spend on one.
To know what WiFi radios your Windows devices support, just run admin command prompt and type "netsh wlan show driver"
Hope this helps
C:\windows\system32>netsh wlan show driver
Interface name: Wi-Fi
Driver : Marvell AVASTAR Wireless-AC Network Controller
Vendor : Marvell Semiconductor. Inc.
Provider : Marvell Semiconductor. Inc.
Date : 6/4/2015
Version : 15.68.3091.193
INF file : C:\windows\INF\oem80.inf
Files : 3 total
Type : Native Wi-Fi Driver
Radio types supported : 802.11ac 802.11n 802.11b 802.11a 802.11g
FIPS 140-2 mode supported : Yes
802.11w Management Frame Protection supported : Yes
Hosted network supported : Yes
Authentication and cipher supported in infrastructure mode:
Authentication and cipher supported in ad-hoc mode: