Printing 2x slower after upgrading to "MarlinTarantula" firmware
First, Ultimaker Cura estimated printing times are not necessarily accurate for non Ultimaker printers. For Ultimaker printers this is perfectly tuned, Cura estimates the printing times from experience for the UM3E correctly. Please look into question "How to determine real printing speed (tarantula/cura)?" which is relevant to this question and has an interesting accepted answer.

Second, changing the MAX accelerations will not work, you should increase the default acceleration:

From the TEVO Tarantula Marlin fork you read from the Configuration.h file:


/**
 * Default Acceleration (change/s) change = mm/s
 * Override with M204
 *
 *   M204 P    Acceleration
 *   M204 R    Retract Acceleration
 *   M204 T    Travel Acceleration
 */
#define DEFAULT_ACCELERATION          1000    // X, Y, Z and E acceleration for printing moves
#define DEFAULT_RETRACT_ACCELERATION  2000    // E acceleration for retracts
#define DEFAULT_TRAVEL_ACCELERATION   3000    // X, Y, Z acceleration for travel (non printing) moves


From the main Marlin source you will find:


/**
 * Default Acceleration (change/s) change = mm/s
 * Override with M204
 *
 *   M204 P    Acceleration
 *   M204 R    Retract Acceleration
 *   M204 T    Travel Acceleration
 */
#define DEFAULT_ACCELERATION          3000    // X, Y, Z and E acceleration for printing moves
#define DEFAULT_RETRACT_ACCELERATION  3000    // E acceleration for retracts
#define DEFAULT_TRAVEL_ACCELERATION   3000    // X, Y, Z acceleration for travel (non printing) moves


The TEVO Tarantula sources have lower default acceleration values, you could try to increase those.