You need to read the fine print when it comes to those high speed packages. The 20Mbps are mostly guaranteed between the customer and their data centres but the moment you hit international waters contention kicks in. 

Even the zuku does it read part of their fair usage policy

" You can subscribe one of three Internet bandwidth options – 1Mbps, 8Mbps or 20Mbps. The bandwidth speed quoted in Tariff Guide and on your bill is the downstream local bandwidth between Zuku datacentres located in Kenya and the Customer’s site. Unlike with many broadband services, ZUKU’S LOCAL BANDWIDTH IS NOT SHARED OR CONTENDED. You are able to enjoy the maximum speeds at the quoted Internet bandwidth to download content that is hosted at Zuku datacentres and we do not limit the amount of data, or the speed at which you download any content between the Zuku datacentres in Kenya and Customer’s Site. For example, if one person at the Customer’s Site is watching a YouTube video, it will stream down to that person at the maximum speed of your connection i.e. 1Mbps, 8Mbps or 20Mbps (because YouTube is localized at Zuku datacentres in Kenya). However, if 4 household members at the Customer’s Site watch different YouTube videos at the same time, each will get 0.25Mbps, 2Mbps and 5Mbps respectively for subscriptions of 1Mbps, 8Mbps or 20Mbps Internet access component of Zuku Triple-play service. Therefore, the higher the Internet speed subscribed to, the better the experience for each user sharing a single Internet connection at the Customer’s Site.

As is the case with all broadband services, users of Zuku share the available international bandwidth capacity that connects them to Internet content located outside Kenya via the more expensive international fibre optic links. ZUKU’S INTERNATIONAL BANDWIDTH IS CONTENDED BY ALL USERS IN PROPORTION TO THE QUOTED LOCAL BANDWIDTH SPEED. The sharing of Zuku international bandwidth capacity is designed to deliver the best possible residential user experience and quality of service to all residential users in a fair and equitable manner.

The Fair Use Policy is a means to monitor and control Zuku international bandwidth capacity to give all users fair access to the available capacity. The Fair Use Policy ensures that this international bandwidth capacity is not used disproportionately by some and others are penalized on their account. Zuku operates a Fair Use Policy, for users who have downloaded an excessive volume of international data in course one day’s use, which can limit the connection speed – during heavy peak hours – in order to ensure that others who have not downloaded their fair share of international data on that day are not affected."