Instead of adding sessions through the UI, you can also add them via CSV upload. Here's how you do it.

Go to Schedule and click Download CSV file template.
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The CSV upload process overwrites what's been created already. Therefore, it's important to start with what you've already added if you want to keep it.

In the Schedule module, select export schedule CSV, which is next to download CSV file template.
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The CSV file format has 14 columns each representing a session's property. Each session gets its own row.

Title — Name of the session

Description (optional)  — Additional information about the session

Event start date  — Date when the session starts, e.g. "31/10/2019"

Event start time — Time when the session starts, e.g. "13:00"

Event end date — Date when the session ends, e.g. "31/10/2019"

Event end time — Time when the session ends, e.g. "14:00"

Location (optional) — e.g. "Room 42" 

Presenter (optional) — e.g. "John Doe"

Bookable (optional) — e.g. TRUE or FALSE
TRUE if you are using Personalized Agenda. Otherwise, leave empty. 

Featured (optional) — e.g. TRUE or FALSE
If set to "true" the session will be highlighted, otherwise leave empty

Color (optional) — Use HEX color code, e.g. "#4286f4"
Highlight each session with a custom color. Make sure you pick something compatible with your theme. Leave empty to let the session follow the meeting theme automatically. 

Capacity (optional) — Attendee cap, e.g. "30"
Put a cap on the number of attendees who can add the bookable session to their personalized agenda. When the limit is reached attendees will no longer be able to book that session and add it to their personalized agenda.  Leave empty to allow an unlimited number of attendees. 

groupid  — Gather multiple sessions in a group, e.g. "1", "3"
Used for bookable sessions, each attendee can select only one session from that group. This is great for when you have the same class repeated at different times. To avoid any errors with the schedule, the groupid column should not be empty if you have more than 2-3 rows in the CSV file. If you don't have any groups in the schedule, then just add an series of incrementing numbers in the groupid column. 

bookingid (leave it empty) — This is a code generated by the system after a bookable session gets created.
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You'll need this code if you want to do pre-booking of sessions to build different agendas for multiple groups of your audience.
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Now that you've created your schedule CSV, it's time to upload it. Remember, the upload overwrites every session in your meeting.
Again, go to the Schedule tab in Presenter view. This time click the UPLOAD SCHEDULE CSV button.
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Check the schedule in Presenter view and in attendee view to ensure it's what you want. If there are only slight changes to be made, you can edit each session manually.
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Here's an example schedule file:

The class "Great Q&A" is offered at 3 different times, so those sessions are grouped together. That way each attendee only signs up for the session once. The Meet & Greet is a key event, so it's featured. All the sessions will follow the theme's color automatically.
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And this is what it looks like in Presenter view:

Finally, this is what it looks like in attendee view.