There are various factors that impact on your data speeds - Distance from the three network mast(s), as three is the parent company of Smarty. Also the types of buildings you live in and around you affect speeds, as they be creating a faraday cage, and reducing signal / speeds.
Also, be aware of section 5.15 of the Smarty terms and conditions :
Responsible use โ Unlimited Data Only Plans
5.15 Our Unlimited Data Only SIM plans are for personal, non-commercial use only and can be used in SIM enabled tablets or
laptops and portable mobile WI-FI devices and is uncapped for this use. This SIM card should not be used as a permanent
long-term alternative to home broadband. If we determine in our sole opinion that your use of the SMARTY Services is in
breach of these Terms and Conditions and/or exceeds that reasonably expected of someone using the Service for legitimate
personal, non-commercial use, then we may at our discretion terminate (in accordance with Section 9), or Suspend (in
accordance with Section 8), or restrict your SMARTY Service. As an example, without limitation, we consider that if you
regularly tether more than 8 devices that this is unlikely to be legitimate personal, non-commercial use.
