Since every wireless router will negotiate wireless speed with the clients down to 1 Mbps if the radio link is of very bad quality, nobody can help you if you just state "WiFi is too slow".

Much more information would be needed to give an advice. Need to know: router model, hardware version, firmware version, distance to router, frequency band, WiFi channel used, number of neighbor's APs interfering, client hardware, obstacles between clients and router, and so on.

jackmarco wrote

Use a free program called Wi-fi History View to review each device that has connected to your network, and look for IP addresses you don’t recognise.

WiFi History View just can show you wireless connections to a wireless network on your own computer. It can not show wireless connections to an external wireless router.

Of course, yes, you should always use a strong WPA2 key (strong = at least 12 to 16 characters in length) to secure your wireless network. But this does not eliminate the need to set up a router so that it can negotiate best possible wireless speed between the WiFi router and a client device. If the WiFi signal is distorted because of too much attenuation or interference, you will notice either no WiFi signal at all or a very low throughput down to 1 Mbps which would lead to ~500 Kbs of data throughput only. Nothing unusual - that's how WiFi networks are supposed to work.

Therefore, any information about your WiFi installation is needed first to be able to give advice how WiFi throughput can be raised.