You've given very little information about where your connection comes from; which country you're in, which version of iOS, connection provider etc. So it's difficult to be specific but here's a few things to consider/try.
Restart your phone and Mac.
Get the Speed test app installed on your phone. Run the test. Look at the results.
If you get slow speeds it's your mobile data connection that's the issue rather than the connection between phone and Mac.
To tether the iPhone (in most countries) you need a contract/tariff that permits tethering or a paid add-on specifically for tethering.
In many cases this data is limited in quantity (500mb per month, 1gb, 2gb etc). Often once this allowance is used up providers, rather than severing the data completely will restrict the bandwidth.
Also if a provider recognises that tethering is being attempted when the contract/tariff doesn't permit it the connection may be restricted.
Again this is all educated guessing without further, specific, information.
You may also be experiencing an outage at a cell tower, so a look at the provider Service Status page of the website could be helpful.
Finally, once you're happy your hardware is performing OK and that you should be getting a stronger connection a call to your phone provider support may well be the order of the day.